Add color support for `-progress` output

[mfridman]

Picking up from #95 (comment), let's add color support to the package -progress output.

Outside the scope of this PR, but as we flush out the functionality of -progress, we should also consider adding artificial output periodically.

From #86 (comment):

For these cases, I think a solution may be to add a -periodic flag which can be set to an arbitrary duration 1m and it'll guarantee to output something to inform CI that tests are still running. Maybe this could default to 1m when -progress is true. This is similar to what hashicorp/terraform#6163 does.

Instead of another flag, maybe -progress could just output something on an interval by default.